Class D amplifiers are popular for signal amplification due to their high efficiency at full power, reduced need for heat sinking, and smaller quiescent currents. Traditional low-voltage (i.e., less than 5V) class D architectures use an n-channel metal-oxide-semiconductor (NMOS) field-effect transistor (FET) on a low side and a p-channel MOS (PMOS) FET on a high side.
NMOS FETs are appealing to use on the high side of the class D amplifier due to their lower drain-source resistance (RDSon) and higher switching speeds, but they require the gate voltage to be higher than the supply voltage. Such a gate voltage is typically provided by bootstrap circuits, which are often used to provide the floating power supply for the high-side switch gate drive. However, bootstrapping in an integrated circuit (IC) often requires extra pins on the IC and components external to the integrated circuit, which raises complexity and cost. Bootstrapping may also be limited by the frequent charging time requirements that conflict with the gate drive operation, especially under varying duty cycle conditions. If the bootstrap capacitor is not adequately charged, the high-side gate drive may even be completely starved of energy and be, therefore, unable to turn on.